I've already found gold, iron, diamonds, etc. But with what material should I build my weapons / armor? What is better or stronger?
Obsidian, but you can't build objects with it
The strongest material to make weapons and armor is diamond. But you can reinforce it by enchanting it
Weapons of wood, armor of leather. Otherwise you will easily hurt yourself while playing.
Strongest unsustainable material in the singelplayer would be Bedrock / Enderportalrahmen / Barrierenblock, then Obsidian which you can break down. But you can't make anything with them.
The best material for making tools and armor is diamond. But you can still enchant the tools / armor.
The leather armor is rarer in the single player than the diamond armor.
